Alzheimer's disease (AD) is characterized by hallmark lesions, such as amyloid rich senile plaques (SP)s), other amyloid deposits, and neurofibrillary tangles (NFTs) which accumulate in selected telencephalic regions that are especially vulnerable to degeneration. Recent studies have led to the identification of some of the major components in these lesions. Specifically, abnormally phosphorylated tau proteins (A68, PHF- tau, tau-PHF) are the major components of paired helical filaments (PHFs), the building blocks of NFTs. Additionally, Beta amyloid peptide or A4 (BetaA4) and other Beta amyloid precursor protein (APPs) domains are major components of AD Sps. Increasing evidence favors altered expression or processing of APPs as a mechanism leading to the deposition of BetaA4 in Sps. In preliminary studies, we have shown that fully differentiated, postmitotic human CNS neurons (designated as NT2N cells) derived from a teratocarcinoma cell line, Ntera 2 (i.e. NT2), generate high levels of C- terminal fragments containing full length BA4 from endogenous APPs. The BetaA4 was found within NT2N cells as well as the culture medium. These findings suggest that the normal constitutive processing of APPs in neurons could lead to extracellular amyloid deposition. Other recent studies also demonstrated that A68 injected into rat brain elicits the co-deposition of BetaA4, suggesting an interaction of A68 with BetaA4 in vivo. This grant proposal describes timely experiments to test the hypothesis that: Perturbation of normal APP processing in neurons leading to increased secretion of BetaA4 may result in amyloid formation. This will be tested by the disruption of the neuronal cytoskeleton, inhibition of axonal transport, perturbation of protein phosphorylation and the induction of excitotoxicity with glutamate in an in vitro culture model comprised of NT2N cells. The effects of these perturbations on APP processing, BetaA4 secretion as well as on the interaction of fetal tau and APP fragments will be assessed. The completion of the studies proposed here will not only test the proposed hypothesis, it will also provide important information on the mechanism of amyloid processing, BetaA4 secretion, and the relationship between A68 and BetaA4 during neurodegeneration.